1. Field of the Invention
The invention relates to a control device for a low-pressure fluorescent lamp.
The electrical behavior of fluorescent lamps containing low-pressure gases is similar to that of a zener diode (or avalanche diode), with a resistance in the gas that becomes very low and negative after breakdown. Ions driven at high speed take the atoms of the gas into excited states in which they send out luminous streaks.
2. Discussion of the Related Art
The means used to control these lamps consist of a control device with current source and an oscillating circuit in which the lamp is placed. This oscillating circuit typically has an inductor and a first capacitor that is series-connected with the lamp as well as a second capacitor that is parallel-connected with the lamp. Through this system, current discharges can be made to pass between the two electrodes of the lamp in both directions.
Conventionally, the current source control device has two electronic change-over switches using power transistors supplied by a DC high voltage and a current transformer. The current transformer is preferably a saturation transformer that limits the current in the lamp by the saturation of its core and causes the switching over of the change-over switches.
The electronic change-over switches generally use power transistors based on bipolar technology for the switch-over function and reverse-biased, parallel-connected diodes to conduct the current during the alternations, and various protection elements.
These transformer-based devices require much space and are costly because they require many components and permit only small-scale integration.
To overcome these drawbacks of costs and small-scale integration, control devices without transformers have been developed. These devices illustrated in FIG. 1 comprise two change-over switch or selector switch circuits Com.sub.a and Com.sub.b series-connected between a high supply voltage and the ground. Each change-over switch circuit has a power transistor T.sub.a, T.sub.b with a diode D.sub.a, D.sub.b parallel-connected in reverse forming the change-over switch, and a control circuit CC.sub.a, CC.sub.b to control the gate of the transistor. The diode is generally a parasitic diode of the transistor.
The control circuits CC.sub.a and CC.sub.b comprise a circuit for the detection of the voltage at the terminals of the diode D.sub.a, D.sub.b to activate the ON state of the transistor T.sub.a, T.sub.b when this voltage is zero and a circuit to measure the current flowing into the transistor T.sub.a, T.sub.b to activate the OFF state of the transistor when the integral of this current is greater than a reference value of current.
The principle of operation of the control device is as follows: it is assumed that, when the system is started up, the change-over switch circuit Com.sub.a is closed, i.e. that the transistor T.sub.a is on and that the change-over switch circuit Com.sub.b is open.
The change-over switch circuit Com.sub.a therefore lets through the current flowing in the lamp F and measures this current. When it detects the fact that sufficient current has gone into the transistor T.sub.a, it goes into an open state. The voltage at the terminals of the inductor gets reversed and the current of the lamp which has to continue to flow somewhere then goes into the reverse-mounted diode D.sub.b of the second change-over switch circuit Com.sub.b. The passage of the current into the diode D.sub.b causes a drop in the voltage at its terminals. This drop in voltage is then detected by the control circuit CC.sub.b and the transistor T.sub.b comes on. At the same time, the inductor of the oscillating circuit gets demagnetized. When it is totally demagnetized, the capacitor Cs gets discharged into the lamp F and the current going into the lamp flows in the other direction.
The control circuit thus formed consists of two change-over switch circuits that work independently. For, each circuit detects a drop in voltage at its terminals, goes into the closed state and then, in detecting that sufficient current has passed between its terminals, goes into the open state.
At the resonance frequency f.sub.0 of the system formed by the oscillating circuit and lamp, it can be seen that the change-over switch circuits are opened successively for a half-cycle. In this type of device, it is therefore not necessary for the two change-over switch circuits to work independently.